Layered heaters are typically used in applications where space is limited, when heat output needs vary across a surface, where rapid thermal response is desirous, or in ultra-clean applications where moisture or other contaminants can migrate into conventional heaters. A layered heater generally comprises layers of different materials, namely, a dielectric and a resistive material, which are applied to a substrate. The dielectric material is applied first to the substrate and provides electrical isolation between the substrate and the electrically-live resistive material and also reduces current leakage to ground during operation. The resistive material is applied to the dielectric material in a predetermined pattern and provides a resistive heater circuit. The layered heater also includes leads that connect the resistive heater circuit to an electrical power source, which is typically cycled by a temperature controller. The lead-to-resistive circuit interface is also typically protected both mechanically and electrically from extraneous contact by providing strain relief and electrical isolation through a protective layer. Accordingly, layered heaters are highly customizable for a variety of heating applications.
Layered heaters may be “thick” film, “thin” film, or “thermally sprayed,” among others, wherein the primary difference between these types of layered heaters is the method in which the layers are formed. For example, the layers for thick film heaters are typically formed using processes such as screen printing, decal application, or film dispensing heads, among others. The layers for thin film heaters are typically formed using deposition processes such as ion plating, sputtering, chemical vapor deposition (CVD), and physical vapor deposition (PVD), among others. Yet another series of processes distinct from thin and thick film techniques are those known as thermal spraying processes, which may include by way of example flame spraying, plasma spraying, wire arc spraying, and HVOF (High Velocity Oxygen Fuel), among others.
In many heating applications, a constant temperature across or along a heating target, e.g., a part such as a pipe or an outside environment to be heated, is often desired in order to maintain relatively steady state conditions during operation. For example, a constant temperature along a hot runner nozzle for injection molding equipment is desirous in order to maintain the molten resin that flows within the nozzle at a constant temperature and optimum viscosity for processing. However, each end of the hot runner nozzle presents a local heat sink relative to the overall hot runner nozzle. One end is connected to a manifold, which draws more heat away from the heater, and the other end, the tip, is exposed to the injection cavities/dies, which also draws more heat away from the heater. As a result, non-uniform heat transfer to the molten resin often occurs along the length of the hot runner nozzle, which translates into non-uniform temperature distribution and non-uniform viscosity of the molten resin. When the molten resin has a non-uniform temperature distribution, the resulting injection molded parts often contain defects or may even be scrapped. Increased machine cycle time can also be a result thereof.
To address this problem, existing prior art hot runner nozzle heaters have been designed with a higher watt density local to the ends of the hot runner nozzle to compensate for the heat sinks. Although the heat sinks are somewhat compensated for with the local higher watt densities of the heater, the temperature distribution along the hot runner nozzle still does not achieve a constant level and thus temperature variations remain in the molten resin, resulting in a less than optimal process. Additionally, existing prior art hot runner nozzle heaters typically have no means to compensate for variable heat sinks that exist within a multiple-drop cavity system nor inherent variations due to manufacturing tolerances of the nozzle bodies themselves.